Method of and mechanism for classifying finely comminuted material



Oct. 27, 1942. R. c. 'rHoMPsoN 2,300,324 METHOD 0F AND MECHANISM FoncLAssIFYING FINELY coMMINu'rBD MATERIAL Filed April 27, 1940 www HHH

mi; L.; L.; [..L lm- LJ g W JT (2) Patented Oct. 27, 1942 METHOD OF ANDMECHANISM FOR CLAS- n ING FINELY COMMINUTIED MATE- Riohard Thompson,Cohasset. Mass., assignor to Sturtevant Mill Company, Boston, Mass., acorporation otMassachusetts Application April 27, 1940, Serial No.332,019

14 Claims.

`This invention relates to a method of and apparatus for sorting out orclassifying from finely comminuted or pulverized material particles of asize in the order oi magnitude of one or two microns or less indiameter, these particles being separated from particles of larger sizeup to say 100 microns or so.

Heretofore it has been impossible to separate particles of such smallsize when in dry condition. Wet separation is objectionable since theproduct must be drled-and usually has a tendency to cake on drying, thislargely defeating the purposes oi' the fine separation. Another methodwhich has previously been used to separate iine'particles is a processof centrifuging in an air stream, but this process proves to beunsatisfactory when the particles to be classified are very line, forinstance, are of colloidal iineness, such as exhibited by kaolins. Suchvery fine particles do not respond to this method owing to the occludediilm of air which surrounds every solid object. In objects ofcomparatively large sizes, this film of air is so thin as to have littleor no eilect on the'characterictics of the object itself, but in objectsof very small size the film, which is of constant thickness regardlessvof the size of the object, ma'y be of greater thickness than thediameter of the object itself. In such cases, the mass of occluded lmmay be greater than that of the object so that two particles may havenearly the same effective specific gravity even though the actualdensities of the solid material of the particles diii'er considerably.Similarly, when two particles are so small that the occluded film of airhas a thickness as great or greater than the particles themselves, therespective sizes of the -particles do not greatly affect the size of thespheres which include not only the particles but the air compartmentsabout them.

It is an object of the present invention to provide a method of andapparatus for separating or classifying in a dry condition particles ofsizes much smaller than can be classified by centrifugal means.According to the invention, electric sparks are discharged through athin layer of finely pulverized material so as to blast the individualparticles in the mass awayl from each other and at the same time tobring all of such particles to the same potential. These particles arethen subjected to a eld of electric potential such as causes the finerparticles to move in one direction and the coarser particles to move inanother direction. The finer particles are then collected by means to bedescribed hereinafter,

the coarser particles being collected by other means.

For a more complete understanding of the invention, reference may be hadto the following description of certain embodiments thereof, and to thedrawing of which Figure 1 is a plan view of apparatus embodying theinvention.

Figure 2' is a longitudinal sectional view of the same.

Figure 3 is an elevation of one of. the sparkdischarging members with asectional showing of la conveyor thereunder.

Figure 4 is a diagrammatic view oi the apparatus, indicating the wiring.y

Figure 5 is a wiring diagram of a modification oi a portion of theapparatus shown in Figure 4.

As shown in Figure 2, the apparatus may comprise a pair of suitableconveyors, which, as illustrated, may take the form of the endless beltsI0 and I2, each oi which consists of a band of sheet ymetal orequivalent electro-conducting material supported by pairs of rolls I4and I6, respectively. A suitable housing Il may enclose the conveyors,this housing being fully open at its ends, if desired, as shown. Thelower conveyor I0 has an upper horizontal stretch which is parallel toand spaced from an opposite horizontal stretch of the upper conveyor I2.These conveyors are driven at a relatively Vslow rate in such a mannerthat the opposed stretches move in the same direction. To this end,rolls I4 and I5 at one end Aof each conveyor may be supplied with gearwheels 2'4 and 2 6 meshing respectively with gear wheels 28 and 30, thelatter being mounted on a common shaft 32. As shown, the lower roll I4may also be provided with a worm gear 34 which meshes with a suitableworm 35- on a shaft 38, this shaft being driven by a suitable motor 40through any preferred type of re ducing gears 42. Finely pulverizedmaterial consisting of particles having diameters ranging from, say, 100microns or so down to 1 micron or less is placed in a hopper I44.Material from this 'hopper is forced by a feed screw 48 to dischargeonto the supply end of the beit I0, the feed screw being driven by themotor 40 through a suitable belt connection 48 or otherwise. Thematerial deposited on the belt I0 is spread out into a thin layer, anysuitable spreading means being employed for this purpose. As shown, a4-bladed paddle wheel 50 is mounted so as to extend across the conveyoradjacent to the discharge end oi the feed screw 46. This paddle wheel isrotated by a suitable motor 62 in a direction opposed to the movement ofthe belt I0. This or equivalent spreading mechanism spreads thepulverized material in a thin layer on the surface of the belt I0. Themotion of the belt I carries the layer of pulverized material under asuccession of spark-discharging members 60, six such members beingindicated in Figures 1 and 2. Each of these members may convenientlyconsist of a shaft 62 rotatably mounted above the belt i0 so as toextend across it from one side edge to the other. Suitable means i'orrotating the shafts 62 may be provided, such as pulleys and beltconnections 63 indicated in Figure 1. Projecting radially from the shaft62 are pointed elements 64 which are equal in length andare arranged ina helical series of a single turn extending along the portion of theshaft 62 which is directly above the belt I0, as indicated in Figure 3.By this arrangement, the elements 64 on each shaft 62 successively inturn become the nearest element to the belt i Il, so that when sparkingpotential is impressed on the member 60 during rotation thereof, thesparks passing from the pointed elements 64 to the belt progress acrossthe belt from one side edge to the other. The shafts 62 are preferablyrevolved at a relatively high speed compared with the rate ofk progressof the belt so that sparks are discharged from the elements 64 tosubstantially every point of the belt.

Between the opposed stretches of the two conveyors is supported an openscreen consisting preferably of fine piano wire which may be stretchedtightly in a zigzag formation resembling the letter M, as indicated inFigure l. As shown in Figure 2, this screen is preferably nearer to theupper conveyor than it is to the lower conveyor. Between the screen 10and the portion of the stretch adjacent to the discharge end thereof isa suitable grid 12 consisting of spaced rods or wires. By meanshereinafter described, the upper conveyor is maintained at a highpositive electric potential, 8,000 volts being found satisfactory. Theupper screen 10 is maintained at a high negative potential oi. 8,000volts or so. The grid 12 is maintained at a positive potential of 5,000volts or so. The lower conveyor is grounded.

'I'he operation of the apparatus thus far described is as follows.Finely pulverized material is fed onto a metal sheet such as the supplyend of the belt l0 and is spread into a thin layer by the spreader 50.This layer of material passes into the zone of operation of thesuccessive spark-discharging members 60. The sparks which are dischargedfrom the pointed elements 64 through the material to the belt I0 stir upthe fine material into a cloud in the space between the two conveyors,thus deiiocculating the mass, the particles in this cloud being separatefrom each other and of uniform electrical potential. The potentialgradient between the screen 10 and the conveyor I 0 induces charges onthe opposite ends of each particle, the charges on the larger particlesbeing suiliciently separated so that the particles are attracted to theconveyor i0. On the particles of one or two microns diameter or less theinduced charges and the distance between them are so negligible that theparticles migrate at random in the atmosphere between the two conveyors,most of them eventually coming into the field of the screen 10 Wherethey are bombarded by negative ions and receive strong negative charges.They soon wander into the positive field from the belt conveyor i2 andthe electrical part of the invention.

are thus attracted to, and precipitated on, this conveyor. Theseprecipitated particlesl travel with the conveyor i2 until they reach asuitable doctor blade 14 which scrapes them ofi.' so that they fall intoa chute 16 leading to a suitable receptacle not shown. The largerparticles which return to the conveyor I0 travel thereon until removedby a Idoctor blade 1B. The positively charged grid 12 sets up a strongpositive field near the discharge end of the apparatus which preventsmigrating particles from traveling beyond it. Thus, practically no dustemerges from the discharge end of the apparatus, all of the particlesbeing forced to one or the other of the conveyors.

Figure 4 shows a wiring diagram embodying A source of alternatingcurrent is conventionally indicated by the lines 80. The line voltage isstepped up to half the desired D. C. potential by a suitable transformer82, one end of the secondary of this transformer being grounded. Avoltage regulator 83 is preferably provided for the primary. The otherend of the secondary is connected through a pair of rectifier tubes 64to a bank of condensers 86. The filaments of the rectifier tubes areheated by any suitable means such as transformers 88. The positive poleof the bank of condensers is connected to the conveyor belt I2, thenegative pole being connected to the screen 10. The bank of condensersis grounded at its center, as at 90. Arf off-center tap 92 on thepositive side of the bank of condensers is carried to the grid 12.Suitable voltages are thus impressed on the upper conveyor i2, thescreen 10 and the grid 12, the lower conveyor I0 being grounded, as at94. Thus, for example, the upper conveyor may be supplied with apositive charge of 8,000 volts, the screen 10 with a negative charge of8,000 volts, and the grid 12 with a positive charge of 5,000 volts. Itis understood that, while these voltages have been found to besatisfactory in operation, the invention is not to be limited to theseor other particular voltages.

A high-potential alternating current is supplied to thespark-discharging members 60, a potential of 10,000 volts being found tobe satisfactory. For this purpose, a transformer B6 is connected acrossthe supply lines 80 with a voltage regulato'r 98 in the primary circuit.The secondary of this transformer is grounded, the other end of thesecondary being connected to one pole of each of a group of condensersIDU. The other pole of each of these condensers is connectedto the point|02 of a point-to-plate spark gap, the

plate |04 of the spark gap being connected to one of the members fromwhich sparks are discharged to the belt l0. As indicated in Figure 3,each plate |04 may be mounted on the end of a shaft 62 to rotatetherewith, the point |02 which cooperates therewith being stationary. Anextra condenser |06 is connected to the secondary of the transformer 96and is grounded directly without any spark gap. A choking coil 408 maybe connected in the circuit between the secondary of the transformer 96and the condensers in order to prevent feed back of high frequencycurrent. When a discharge takes place from the points 64 to the conveyorbelt a certain amount of rectification of the current results from thispoint-to-plate gap. This rectification is increased by thepoint-to-plate spark gap between i02 and |04. As is well known, when acondenser discharges, it tends to discharge beyond its total capacity,reversing its polarity with the result that the discharge from thecondenser produces a high frequency oscillation, the typical dischargebeing a damped high-frequency wave. However., the impressed voltage o nthe condensers is an alternating current, and a condenser of givencapacity impressed with an alternating current of a given voiage willpass only a fixed amperage. Hence the ndensers |00 serve two purposes.First, they serve to produce the high-frequency oscillations requiredfor the defiocculatioh of pulverized material at the points 64; andsecond, they serve to distribute the flow of current equally between thespark-discharging members S0 so that all six of these members willoperate `satisfactorily at the same time. The extra condenser I06 is notessential to the operation of the apparatus but serves to..improve suchoperation by amplifying the high-frequency oscillations, thusappreciably increasing the output.

' A modified form of wiring for the spark-discharging apparatus is shownin Figure 5. Instead Y of employing a single transformer to supply highpotential for all of the spark-discharging members B0, each of thesememb'ers is supplied from a separate transformer H0. One end of thesecondary of each transformer is grounded, the other end being connectedto a point-to-plate spark gap which in turn is connected to one of thespark-discharging members 80. A grounded condenser |06 is also providedfor each transformer secondary. Such apparatus functions satisfactorilyalthough the current consumption is somewhat higher than in theapparatus indicated in Figure 4.

Some of the factors affecting the operation of the apparatus hereof willnow be indicated. Thus, it is to be observed that the positively chargedgrid 12 defiects therefrom to either the upper or lower belt, as thecase may be, any particles that may .have received an adventitiousthereby, they are apt to carry or convey undesirably large particles tothe upper belt I2. As

positive charge in the course of their passage be-i v tween the belts.Again, the positively charged particles are actively repelled by suchgrid 12 as they approach thereto; and such neutral and/or negativelycharged particles as may land thereon quickly acquire the potentialthereof and are immediately repelled thereby. To be sure, there usuallyis a thin layer or lm of pulverulent materlal on such grid while theapparatus hereof is operating, but such layer or film is negligibleinsofar as concerns effect on the process hereof. It might also be notedsuch layer or film is constantly sloughing cif from the grid 'I2 andrenewing itself in the course of the process hereof.

Not only does the lower belt or conveyor l0 bear a bound charge(positive) induced thereon by the screen 1U but it also-functions as aconductor for the high-frequency discharges from the points of therotary members B0, wherefore, there constantly ensues on such belt ahighly erratic or variegated potential, which, however, constantlyremains on a distinctly positive side.

The rectifying effect arising from the point-ton plate discharge by thepoint-bearing members relative to the belt i0 and the point |02 relativeto the plate |04 is of considerable value in that the effect of suchrectification is always to impress the particles with a negative chargerather than a positive one.

It is to be observed that the spark-discharging members Bil may berotated .in either direction. In any case, however, the speed ofrotation of such members should be low enough so that no appreciableair-currents are generated thereby,

for, should noteworthy air-currents be generated already stated, thepotentials under which the apparatus hereof may be operated are subjectto considerable variation. In general, it may be said thatintensiiication or increase of potentials, especially when impressed onthe screen 10, increases both the size and quantity of materialcollected on the upper belt or conveyor I2.

Apropos of the M-oonflguration of the screen 10, it is to be observedthat such configuration comports with complete coverage of thecrossscctional area of the upper belt I2 while at the same time makingfor the widest possible spaces for the travelof the migrant particlesaround the wires of such screen. xperimentation has shown that a numberof short wires arranged transversely of the upper belt i2 can functionfairly satisfactorily in lieu of the screen 10, but that such expedientis attended by considerable increase in current consumption and by somedecrease in recovery of the fine-particle size fraction as vcomparedwith the results attained when an M-conflgured `screen was employed. Itis possible also to use an X-configured screen and certain otherwire-arrangements in lieu of the M-configured screen 10, but, so far asapplicant has to date determined, the M-coniigured screen 10 functionsmost satisfactorily for the purposes hereof. i

While the drawing shows an endless belt I0 for conveying a layer ofpulverulent material .progressively through the apparatus hereof, otherconveying means can be employed, including, for example, a vibratingplate of the type frequently used for feeding hot materials in thefoodprocessing industry. Indeed, experiments have indicated that suchlast-described '\conveying means can be employed quite satisfactorily inlieu of such endless belt I0 as is specifically illustrated herein.

It is evident that various other modifications and changes may be madein the embodiments of the invention herein shown and described withoutdeparting from the spirit or scope of the invention as defined in thefollowing claims.

I claim:

1. A method of classifying by size the particles of a particulate mass,which comprises deiiocculating said mass while dry by dischargingtherethrough electric sparks capable of blasting it into a cloud ofdiscrete particles, and subjecting the resulting discrete particles to afield of electric potential adapted to cause particles of relativelysmall size to move in one direction and particles of relatively largesize to move in another direction.

2. A method of classifying by size the particles of a particulate mass,which comprises spreading said mass while dry as a thin layer on anelectro-conducting sheet, discharging disruptive electric sparks throughsaid layer to said sheet at substantially all points thereof and therebydisrupting it into a cloud of discrete particles, and maintaining anelectric field adjacent to said sheet adapted to cause particles ofrelatively small sze. which have been disturbed by said spark discharge,to move away from said sheet and to cause particles vof relatively largesize to return to said sheet.

3. A method of classifying by size the particles of a particulate mass,Vwhich comprises deflocculating said mass while dry by dischargingtherethrough disruptive oscillating electric sparks and therebydisrupting it into a cloud of discrete particles, selectively actingupon the particles of relatively small size with a field of electricpotential to separate them from the particles oi' relatively large size,negatively charging the separated particles of relatively small size,attracting said charged particles to a positively charged member, andcollecting said particles from said member.

4. Apparatus for classifying by size the particles of a particulate masswhile dry, which comprises an electro-conducting sheet adapted tosupport a layer of the mass to be classified, a plurality of pointeddischarge elements above said sheet, means for progressively moving saidlayer relative to said elements so as to bring said elementssuccessively into spark-discharging relation with said layer atdifferent points, and means for impressing on said elements ahighfrequency alternating current of suitable voltage for the dischargeof sparks to said sheet.

5. Apparatus for classifying by size the particles of a particulate masswhile dry, comprising a conveyor consisting of an electro-conductingband having a horizontal stretch, means for driving ,said conveyor,means for spreading finely comminuted material in a thin layer on saidconveyor, spark-discharging means above said conveyor for defiocculatingthe commlnuted material carried thereby, an upper conveyor above saidspark-discharging means consisting of an electro-conducting band havinga horizontal stretch spaced above and' substantially parallel to thehorizontal stretch of the first-mentioned conveyor, means formaintaining a positive potential on said second conveyor, an open screenbetween said conveyors, and means for maintaining a negative potentialon said screen.

6. In an apparatus for classifying finely divided particles, means fordefiocculating dry finely commlnuted material comprising an endlessmetal band having a horizontal stretch, means for supporting and drivingsaid band, means for supplying material in a layer near one end of saidstretch, a series of spark-discharging members above said stretch, eachsaid member including a shaft extending transversely above said stretchand a series of pointed elements projecting from each shaft and arrangedin a single helical turn whereby said elements are successively in turnnearest to said stretch as the shaft is rotated, and means forimpressing on said members a high potential alternating current which isevenly distributed among the several said members.

7. Apparatus of the class described, comprising a lower and an upperconveyor consisting of metal belts having horizontal stretches onespaced above the other, means for driving said conveyors so that theopposed stretches travel in the same direction, means for feeding to thelower conveyor a layer of finely commlnuted material to be classified,means for discharging sparks through said layer to deflocculate saidmaterial, an open screen between said opposed stretches, means formaintaining high positive and negative potentials on the upper conveyorand screen respectively. and a positively charged grid between saidopposed stretches near the discharge end thereof;

8. Apparatus for deflocculating dry pulverized material, comprising ametal support for a layer of said material, and means for dischargingdisruptive electric sparks to said support and through said layer andthereby disrupting it into a cloud of discrete particles, said meansincluding a pointed element in` spark-discharging relation to saidsupport, a source of high-voltage alternating potential, a condenser,and a point-toplate spark gap connected in series with said condenserbetween said source of potential and said pointed element.

9. Apparatus for deflocculating dry pulverized material, comprising ametal support for a layer of said material, and means for dischargingdisruptive electric sparks to said support and through said layer andthereby disrupting it into a cloud of discrete particles, said meansincluding a plurality of pointed elements in spark-discharging relationto said support, a point-to-plate spark gap connected to each saidelement, a condenser connected to each said spark gap, and means forimpressing a high voltage alternating potential on said condensers.

10. Apparatus for deilocculating dry pulverized material, comprising ametal support for a layer of said material, and means for dischargingdisruptlve electric sparks to said supportand through said layer andthereby disrupting it into a cloud of discrete particles, said meansincluding a plurality of pointed elements in spark-discharging relationto said support, a point-to-plate spark gap connected to each saidelement, a condenser connected to each said spark gap, a transformerhaving one end of its secondary connected to all of said condensers, andan extra condenser connected to said end of the transformer secondaryand grounded.

11. In apparatus of the class described, means for deflocculating anddispersing in the atmosphere the particles to be classified and meansfor classifying them by electrostatic forces, said classifying meansconsisting of a grounded electro-conducting plate and a positivelycharged electro-conducting plate substantially parallel to each otherand containing therebetween a thin negatively charged screen comprisedof wires oi small diameter.

12. In an apparatus for classifying finely divided dry particles ofmatter according to size comprising, in combination, a positivelycharged electro-conducting surface, a grounded electroconductingsurface` said surfaces being substantially parallel with each other, athin negatively charged screen positioned between said surfaces andincluding wires of small diameter, and means for delivering deocculatedparticles to be classified into the weaker electro-static fieldmaintained in the space between said grounded surface and said screen,whereby particles of different sizes are precipitated on said respectivesurfaces.

13. In an apparatus for classifying finely divided dry particles ofmatter according to size comprising, in combination, a positivelycharged electro-conducting surface, a grounded electroconductingsurface, said surfaces being substantially parallel with each other, athin negatively charged screen positioned between said surfaces andincluding wires of small diameter, means for delivering deiiocculatedparticles to be classified into the weaker electro-static fieldmaintained in the space between said grounded surface and said screen,whereby particles of different sizes are precipitated on said respectivesurfaces'means for moving said surfaces in their own planes and in thesame general direction to transfer the particles so precipitated,apparatus for removing the precipitated materials separately from saidsurfaces, and a positively charged grid interposed between said groundedsurface and said negatively charged screen in a region .remote from thatin which said particles are so delivered.

14. In an apparatus for classifying ilnely divided dry particles ofmatter according to size comprising the combination of twoelectro-conducting conveyors having parallel runs spaced apart andpresenting relatively large opposed parsaid screen including relativelysmall wires, means for conducting nely divided material to be classiiledinto the space between said screen and said grounded conveyor where saidparticles will be acted upon by the electro-static eld in said space,means for maintaining said particles in a deocculated state, a positivegrid interposed between said conveyors and parallel to them but locatedin a region spaced from the zone of delivery of said particles into saidspace, and means for removing the material collected on said conveyors.

RICHARD C. THOMPSON.

